1. Field of the Invention
This invention relates to the field of valves, namely plug valves.
2. Background Information
Plug valves are quarter-turn valves with a rotating element that is shaped like a truncated cone. In 1989, a patent was issued, U.S. Pat. No. 4,878,652, Plug Valve, in which the sides of the truncated cone are modified by shaping it in the form of a tractrix curve from top to bottom. The purpose of the curved surface in the form of a tractrix is to impart even and uniform wear between the mating surfaces of the plug and body. As a result, leaks through the seat of the plug valve are eliminated, since the plug continues to wear in against the body, and continues to mate perfectly. Numerous tests with valve models, and subsequently, several valve prototypes have shown this to be a fact.
In 1991, a second patent was issued, U.S. Pat. No. 5,044,606, that used the tractrix technology to create a stem seal for any quarter-turn valve, such as a plug valve, ball valve, or a butterfly valve. Prototype valve design has incorporated the tractrix stem seal into the tractrix valve. This new and unique combination of the tractrix valve with the tractrix stem seal has resulted in simplifications and cost reductions for producing the valve.
The theory of even wear using the tractrix curve is described elsewhere and is a known art. The basis of the plug design is a tractrix body of revolution, known as a pseudo sphere.
Small plug angles are utilized to create large sealing pressure with a small axial force. This occurs due to the nature of the axial pressure, Pa, that is a result of the axial force, Fa, illustrated in FIG. 1. The axial pressure is defined as the axial force, Fa, divided by the projected area, A, of the tractrix plug. The axial pressure is a constant over the entire surface of the tractrix body of revolution. The axial pressure depends on two variables, the axial force and the slope angle of the tractrix curve. FIG. 2 shows that a large force produces a large axial pressure, while a small axial force produces a small axial pressure. The effect of slope angle is shown in FIG. 3, where, for the same axial force, the normal pressure, PN, is much larger for the smaller slope angles of the tractrix curve. In equation form:
PN=Pa/sin xcex8
As can be readily seen, a small angle can produce a large normal pressure that can be utilized for sealing against internal fluid pressures. As a result of this feature of the tractrix curve, smaller plug angles have been used in the design of prototype tractrix plug valves. Slope angles for tractrix plugs have spanned the range from 4 degrees to 15 degrees with one tractrix plug using an angle of 27 degrees. Practical considerations have resulted in a preferred tractrix plug angle of about 8 degrees.
The Tractrix Valve comprising a body, plug with integral stem, stem seal, and bonnet is described. The small end of the tractrix plug fits down inside the body of the valve. This places the large end of the plug near the middle of the valve. Lubrication grooves are located in the plug in order to add lubricant to the plug/body interface. The lubrication reduces friction between the plug and the body in order to keep the torque low. There is an integral stem rigidly attached to the plug. Since the plug and stem must turn on the same axis, it is important that the plug and stem be concentric. A lower portion of the stem is flared in the form of a tractrix shape to accommodate a stem seal with an identical mating tractrix surface forming a stem/stem seal interface. The direction of the tractrix flare on the stem is opposite from the direction of the tractrix flare of the tractrix plug. Usually, the stem seal is made of an elastomeric material, such as TEFLON or polypropylene, selected for low friction as one of its properties. Low friction is important in order to keep the torque required to open and close the valve within a reasonable value. A bonnet is attached to the tractrix valve body by means of a plurality of bolts. The bonnet serves to hold the plug, stem, and stem seal in place. Springs are employed to hold the plug against the body and simultaneously force the stem seal against the stem. A retainer ring transmits the spring force to the stem seal. The stem and stem seal combination doubles in function as a journal bearing. The journal bearing function allows the stem seal to remain fixed while permitting the plug with integral stem to turn to allow the valve to open and close against the axial spring force imposed on the stem seal. The springs are commonly disc springs, also called Belleville washers, but the axial force function could also be generated by other means. The function of the axial force generated by the springs is to hold the plug against the body thereby creating a primary seal against the internal fluid pressure in the valve. The same spring force pushes the stem seal against the stem thereby creating a secondary seal on the stem. It""s necessary for the stem seal to remain stationary so a tight static seal can be maintained between the interface of the stem seal and the valve bonnet.